Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho³⁺ single-molecule magnet

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc₃(THF)₂Li₂]^-, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc^2-) average twist angle with decreasing ionic radius (r _ion) of the central Ln ion, resulting in the largest average Fc^2- twist angles for the Lu³⁺ compound 6. Such high sensitivity of the twist angle to changes in r _ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho³⁺ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110-131 cm^-1. In contrast, compounds featuring Ln ions with prolate electron density (3-5) don't show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2-5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc₃(THF*)₂Li₂]^- (2-THF*) and [HoFc₃(py)₂Li₂]^- (2-py) motifs. We find that larger average Fc^2- twist angles (in 2-THF* and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.